<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }


    #root {
      display: flex;
      width: 100%;
    }

    .start-box {
      /* background-color: brown; */
      width: 300px;
      height: 100px;
      display: flex;
      /* overflow: auto; */
    }

    .start-box>.item {
      /* flex: 1 0 100%; */
      height: 100%;
      flex: auto;
      /* flex-shrink: 0; */
      /* width: 100%; */
    }

    .start-box>.item:first-child {
      background-color: red;
      width: 100px;
    }
  </style>
</head>

<body>
  <div id="root">
    <div class="start-box">
      <div class="item">1



      </div>
      <div class="item">2</div>
      <div class="item">3</div>
    </div>
    <!-- <div class="center-box">
      <div class="item"></div>
      <div class="item"></div>
    </div>
    <div class="end-box">
      <div class="item"></div>
      <div class="item"></div>
      <div class="item"></div>
    </div> -->
  </div>

</body>

</html>